Fertilisation And Implantation PDF Year 11 Human Biology

FERTILISATIO N AND IMPLANTATIO N Year 11 Human Biology  Upon ejaculation, sperm are released into the vagina at the cervix (entrance to the uterus).  Known as insemination. Sperm travel through the cervix FERTILIS and bod...

FERTILISATIO N AND IMPLANTATIO N Year 11 Human Biology  Upon ejaculation, sperm are released into the vagina at the cervix (entrance to the uterus).  Known as insemination. Sperm travel through the cervix FERTILIS and body of the uterus and into the fallopian tubes. ATION  Inside fallopian tubes within minutes – propelled by muscular contractions of uterus and fallopian tubes. Fertilisation usually occurs when the ovum is 1/3 of the way down the fallopian tube.  Secondary oocyte (released at ovulation) is surrounded by 2 layers;  Corona radiata – outer layer of follicle cells held together by an acid.  Zona pellucida – inner layer of glycoprotein surrounding oocyte’s plasma membrane. FERTILIS Enzyme contained in sperm acrosome breaks down the acid holding the ATION corona radiata together.  Several thousand sperm required to loosen the cells and allow the entry of one sperm. Sperm then encounter zona pellucida - release digestive enzymes to breakdown the glycoprotein matrix.  Single sperm cell able to enter the ovum - initiates formation of a fertilisation membrane.  Plasma membrane of sperm and ovum fuse and nucleus of sperm (male pronucleus) moves through cytoplasm. Sperm entry stimulates secondary oocyte to finish the second meiotic division and nucleus becomes the female pronucleus.  Male and female pronuclei fuse to form a single nucleus with FERTILISATION diploid chromosome number.  Now known as a zygote. FERTILISATION  After fertilisation, the zygote travels down the fallopian tube and begins to divide by mitosis. Cleavage – rapid cell division that results in a multicellular embryo.  Morula formed during cleavage phase (day 3-4) IMPLANT By 6 days after fertilisation, a blastocyst has formed that has now reached the uterus. ATION  Blastocyst – hollow ball of cells surrounding a cavity filled with fluid. Inner cell mass – Structure of about 30 pluripotent stem cells on one side of the cavity in the blastocyst. Blastocyst - free in the uterus for 2-3 days before sinking into endometrium and becoming firmly attached to the wall of the uterus (implantation). IMPLANTATION  During implantation, the inner cell mass undergoes changes to form 3 layers of cells (primary germ layers). Primary germ layers will differentiate into all the tissues that make up the body.  Ectoderm – Outer layer. Gives rise to the skin and the brain and nervous system.  Mesoderm – Middle layer. Gives rise to skeletal and muscle tissues, and heart and circulatory system. PRIMARY GERM  Endoderm – Inner layer. Gives rise to inner lining of LAYERS many organs and the digestive system. PRIMARY GERM LAYERS  Early in embryonic development, 4 embryonic membranes develop.  Lie outside the embryo and protect and nourish it during development. Amnion – First to develop. Surrounds EMBRYO the embryo by 8th day after fertilisation.  Produces amniotic fluid – shock absorber, NIC maintains constant temperature and MEMBRA allows movement of embryo/foetus. Chorion – Formed from outer cells from NES the blastocyst and a layer of cells from the mesoderm.  Surrounds the embryo and other membranes and becomes the foetal part of the placenta. Two other membranes, not as important, become part of the umbilical cord. EMBRYONI C MEMBRAN ES